1. Field of the Invention
This invention relates generally to giant magnetoresistive (GMR) spin valve (SV) sensors and more particularly to a high-sensitivity self-aligned lateral current-perpendicular-to-plane (CPP) dual-SV sensor geometry and fabrication method.
2. Description of the Related Art
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read. A direct access storage device (DASD) or disk drive incorporating rotating magnetic disks is commonly used for storing data in magnetic form in concentric, radially-spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive (MR) read sensors (MR heads) are preferred in the art because of their capability to read data at greater track and linear densities than earlier thin film inductive heads. An MR sensor detects the magnetic data on a disk surface through changes in the MR sensing layer resistance, which are responsive to changes in the magnetic flux sensed by the MR layer.
The early MR sensors rely on the anisotropic MR (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetic moment of the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) changes the moment direction in the MR element, thereby changing the MR element resistance and the sense current or voltage.
The later giant magnetoresistance (GMR) sensor relies on the spin-scattering effect. The chief source of the GMR effect is “spin-dependent” scattering of electrons. In GMR sensors, the resistance varies as a function of the spin-dependent scattering of the conduction electrons across two magnetic layers separated by a non-magnetic spacer layer. The spin-dependent scattering occurs at the interface of the magnetic and nonmagnetic layers and within the magnetic layers. Electrical resistance is affected by scattering of electrons moving through a material. Depending on the direction of its magnetic moment, a single-domain magnetic material scatters electrons with “up” or “down” spin differently. When the free and pinned magnetic layers in a GMR structure are aligned anti-parallel, the resistance is high because “up” electrons that are not scattered in one layer may be scattered in the other. When the layers are aligned in parallel, scattering is reduced for all of the “up” electrons, regardless of which layer they pass through, yielding a lower resistance. GMR sensors using only two layers of ferromagnetic (FM) material separated by a thin layer of non-magnetic conductive material (e.g., copper) are generally referred to in the art as spin valve (SV) sensors.
The sense current-in-plane (CIP) SV sensor is well-known in the art and includes a nonmagnetic electrically conductive spacer layer sandwiched between a FM pinned layer structure and a FM free layer structure. An antiferromagnetic (AF) pinning layer interfaces the pinned layer structure for pinning a magnetic moment of the pinned layer structure 90E to an air bearing surface (ABS), which is an exposed surface of the sensor that faces the magnetic disk. Two sense current lead conductors are connected on each side of the layered SV structure to conduct sense current in the plane of the several layers. The magnetic moment of the free layer structure is free to rotate upwardly and downwardly with respect to the ABS from a quiescent position or bias point in response to positive and negative magnetic field signals present on the surface of an adjacent rotating magnetic disk. The quiescent position, which is preferably parallel to the ABS, is the position of the magnetic moment of the free layer structure with the operating-bias sense current conducted through the sensor in the absence of external magnetic fields.
The spacer layer thickness is chosen to minimize the shunting of the CIP sense current and the magnetic coupling between the free and pinned layer structures. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons are scattered at the conductive spacer layer interfaces with the pinned and free layer structures. Such scattering is minimal when the pinned and free layer magnetic moments are parallel with one another, and increases substantially when the magnetic moments are antiparallel. Because changes in scattering affects the SV sensor resistance, the sensor resistance varies as a weighted function of cos θ, where θ is the relative angle between the magnetic moments of the pinned and free layer structures. SV sensor sensitivity is quantified in terms of the MR coefficient, δr/R, where R is the sensor resistance when the magnetic moments are parallel and δr is the change in the sensor resistance arising from shifting the moments into an antiparallel position.
The sensitivity of a SV sensor depends upon the response of the free layer to external magnetic field signals from the surface of a rotating magnetic disk. The magnetic moment of the free layer depends upon the material or materials employed for the free layer. The responsiveness of the free layer decreases as the magnetic moment of the free layer increases. Reduced responsiveness means the free layer magnetic moment cannot rotate as far from its parallel position to the ABS for a given external magnetic field level, which reduces sensor signal output. Also, improved isolation of the free layer structure from the pinned layer structure usually requires a thicker intermediate conductive layer, which shunts sense current away from the FM layers, thereby reducing sensor resistance and sensitivity.
For example, FIG. 1 shows an ABS view (disposed for vertical relative medium motion) of a typical CIP SV sensor 20 from the prior art that is stabilized using the hard magnetic (HM) layers 22 formed by a lift-off process. SV sensor 20 is usually fabricated using thin-film deposition techniques known in the art. For example, a first shield (S1) layer 24 of a conductive material is formed on a substrate (not shown) and an insulating layer 26 of alumina, or the like, is deposited over S1 layer 24. The SV layers are then deposited in sequence over insulating layer 26. For example, the AFM pinning layer 28 is deposited followed by the FM pinned layer 30 to form a pinned layer structure. Next, the conductive spacer layer 32 of copper, or the like, is deposited followed by the FM free layer 34. Finally, a photoresist layer (not shown) is formed over the entire assembly and is processed in the usual manner to permit all material outside of the central (read-width) region 36 to be removed by etching down to insulating layer 26 (the “track-mill” step). After etching, a HM material is deposited over the exposed portions of insulating layer 26 and also over the remaining photoresist layer (not shown) in central region 36 and, before removing the photoresist layer covering central region 36, a conductive lead layer 38 is deposited over everything. The photoresist layer is then finally dissolved away, which “lifts off” the unwanted portions (not shown) of the HM layer 22 and lead layer 26 within central region 36, in a well-known manner. Because of this lift-off deposition procedure, the later layers are tapered to a very slight thickness at the junction with central region 36. The sense current (not shown) flows parallel to the plane of layers 28–34 from one side of lead layer 38 to the other, so the (fixed) conductivity of conductive spacer layer 32 reduces the GMR effect of scatter-dependent conductivity of MR layers 30 and 34.
As an alternative to the CIP structure, the sense current lead conductors may be arranged so that the sensing current passes through the sensor perpendicular to the plane of the layers, which is known in the art as current-perpendicular-to-plane (CPP) geometry. In an early paper, [AA New Design for an Ultra-High Density Magnetic Recording Head Using a GMR Sensor In the CPP Mode,” Rottmayer, R. and Zhu, J.; IEEE Transactions on Magnetics, Vol. 31, No. 6, November 1995], Rottmayer et al. propose a GMR multilayer read element within a write head gap that operates in the CPP mode and is biased by an exchange coupled soft film acting like a permanent magnet while distinguishing conventional MR and SV head designs. Their read element has a repeated multilayer structure (to increase GMR sensitivity) that is quite different from the GMR SV stack later introduced in the art.
For example, FIG. 2 shows a partial ABS view (disposed for vertical relative medium motion) of a typical CPP SV sensor 40 from the prior art. Sensor 40 includes a substrate 42 with an overlying underlayer 44. In turn, an insulating gap layer 46 overlies underlayer 44. A first magnetic shield (S1) layer 48 overlies gap layer 46 substantially as shown. An optional gap layer 50, which may include aluminum-oxide (Al2O3) or silicon-dioxide(SiO2), may be formed upon first magnetic shield (S1) layer 48. The S1 & S2 shield layers may include nickel iron (NiFe), cobalt-zirconium-tantalum (CoZrTa), iron-nitride (FeN) or any other useful soft magnetic materials or their alloys, and may be about 2 microns or less in thickness. Gap layer 50 may be from about 10 to about 100 nanometers in thickness. A first lead (L1) layer 52 is formed on top of gap layer 50. First lead (L1) layer 52 may include between 10 and 100 nanometers in thickness of rhodium (Rh), aluminum (Al), gold (Au), tantalum (Ta) or silver (Ag) or their alloys. A FM free layer 54 overlies first lead (L1) layer 52. A non-magnetic conductive spacer layer 56, usually copper (Cu), overlies free layer 54 and a pinned layer 58 is formed on top of spacer layer 56. A pinning layer 60 overlies pinned layer 58 and a second lead (L2) layer 62, of material similar to that used to produce first lead layer 52, is formed thereon. First and second lead layers 52, 62 in conjunction with free layer 54, spacer layer 56, pinned layer 58 and pinning layer 60 together make up the SV stack 64, substantially as shown. An optional gap layer 66 may overly second lead (L2) layer 62 to isolate therefrom the second magnetic shield (S2) 68. A dielectric gap material 70 surrounds SV stack 64 and portions of first (L1) and second (L2) lead layers 52, 62 substantially as shown. The sense current (not shown) flows perpendicular to the plane of layers 94–100 from one to the other of lead layers 52, 62. The inverted SV structure wherein pinning layer 60 and pinned FM layer 58 underlie active FM layer 54, may be used instead of the more conventional arrangement of active and pinned layers shown.
By passing the sense current through SV stack 64 perpendicularly to the plane of layers 54–60, the spin-dependent scattering effect may be exploited while eliminating the effect of the in-plane current usually shunted through the non-magnetic layers such as conductive spacer layer 56. It has been demonstrated that the CPP GMR coefficient (δr/R) is accordingly larger than the CIP GMR coefficient. However, because the film layers 54–60 are quite thin, they have a low resistance perpendicular to their plane (even with modem read-width and throat-height dimensions of about 500 nm) and the series resistance of first (L1) and second (L2) lead layers 52, 62 significantly reduces the sensitivity of the CPP SV sensor over what could otherwise be available. This is true with read-width (RW) and throat-height (TH) dimensions of about 500 nm each and is likely to remain so until these dimensions are reduced by an order of magnitude.
Practitioners in the art have proposed various useful solutions to the CPP SV resistance problem; some proposing to reduce the lead resistance and others proposing to increase the effective CPP SV stack resistance. For example, in U.S. Pat. No. 6,134,089, Ronald Barr et al. disclose a technique for reducing sense lead conductor resistance to better exploit the little resistance available in the CPP GMR stack. In U.S. Pat. No, 6,198,609, Ronald Barr et al. disclose a fabrication technique for increasing stack resistance by preventing sense current shunting at the edges of the CPP GMR stack, thereby improving sensitivity. In U.S. Pat. 5,668,688, John Dykes et al. propose increasing stack resistance by using two SV stacks in series to provide an enhanced δr/R response said to be twice that seen for CIP SV geometries. Moreover, in U.S. Pat. No. 6,233,125, Kenneth Knapp et al. disclose a CPP MR read sensor that is formed in a groove between two conductors by a method results in the sense current passing twice through the MR thickness, thereby doubling the sensor CPP SV stack resistance. Although these proposals do improve exploitation of the inherently better CPP GMR sensitivity by enhancing CPP resistance in different ways, most of these proposed solutions require much more elaborate fabrication procedures than do the simpler CIP SV sensor.
Thus, the larger CPP GMR effect (δr/R) does not readily result in the expected improvement in GMR sensor signal amplitude over CIP SV geometries because the CPP SV resistances are so much lower than the corresponding CIP SV sensor resistances, leading to lower voltage drops for given sense current amplitudes. These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.